1. Field of the Invention
The present invention relates to a shock absorbing system for the front fork of bicycles. More specifically, the present invention relates to such a shock absorbing system having an exchangeable spring unit formed of elastomeric spring pads.
2. Description of Related Art
Front fork suspensions have been known for motorcycles for a long time and with the invention of Turner U.S. Pat. No. 4,971,344, became practical for use on bicycles, and have since found widespread use, particularly on mountain bicycles. In the suspension of the Turner patent, air pressure is used to adjust the hydraulic fork to compensate for rider weight variations or to produce a firmer or softer ride. However, because the extension damping performance of the suspension is directly related to the amount of air pressure in the system, adjusting of the air pressure to compensate for a rider's weight could adversely affect the extension damping characteristics of the suspension, and no other means existed to vary the performance characteristics of the suspension, nor was the suspension designed to produce differing performance characteristics under different loading conditions apart from an ability to "lockout" low level forces of the type produced by pedaling while allowing the fork to react to high impact forces.
In Turner U.S. Pat. No. 5,186,481, the fork suspension of the above-mentioned Turner patent was improved to enable varying of the preload on a coil-type compression spring, that acts to hold a compression valve plate of a metering valve located between upper and lower hydraulic chambers in a closed position until a predetermined force level is reached at which time it pops open to allow flow through the valve. In particular, by turning of an adjustment rod so as to screw it more or less into the metering valve, the coil-type compression spring is caused to contract or expand, thereby changing the force required to open the compression metering valve, and allowing a wide range of adjustments for adapting the suspension to widely differing circumstances of rider weight and riding experience. However, this adjustability does not affect the suspension beyond setting the threshold level at which compression of the fork will commence.
Other shock absorbing bicycle forks have since been developed which allow for personalized adjustment of the performance characteristics of the fork. For example, Wilson et al. U.S. Pat. No. 5,269,549, discloses a suspension for the front wheel of bicycles in which a spring action is used for shock absorbing purposes and is obtained by a rod-mounted arrangement of stacked elastomeric pads which are disposed between the ends of the fork strut tubes to absorb shocks by deformation thereof. By using different combinations of pads of different durometers, resilience characteristics and/or lengths, the shock absorbing characteristics can be changed in accordance with the rider's weight and the conditions under which the bicycle will be ridden. However, such an arrangement simply is incapable of achieving the same kind of ride as a hydraulic suspension, in that springs shock absorbing characteristics are a function of the distance they are compressed while a hydraulic suspension has a damping characteristic that is a function of the speed at which the shock absorber is compressed, and the mounting rod arrangement of this patent is not designed so as to be able to work in conjunction with a hydraulic damping unit. Additionally, an effective distribution of the compressive loading across the pads cannot be assured with the rod-mounted pad arrangement of this patent and no means exists to adjust the compression spring preload, and thus, the initial compression response characteristic of the fork for any given combination of pads.
Chen U.S. Pat. No. 5,284,352 discloses a compression-adjustable bicycle shock absorbing front fork which, like that of Wilson et al., utilizes a rod-mounted arrangement of interchangeable stacked elastomeric pads, to which a compression coil spring is added. In addition to the adjustability afforded by the ability to change elastomeric pads, the initial compression characteristics and the travel length which the strut can be compressed can be adjusted by rotating of a mounting member which acts to reduce or expand the initial height of the stacked pads and spring. Furthermore, washers are disposed between the rubber pads to distribute the external loading to the pads when the forks struts are compressed. While an improvement over the Wilson et al. suspension in that adjustments can be made without replacing one or more elastomeric pads with others, it still lacks the damping characteristics of a hydraulic suspension, and like the Wilson et al. mounting rod arrangement, the mounting rod arrangement of this patent is not designed so as to be able to work in conjunction with a hydraulic damping unit. Additionally, the mounting rod arrangement of elastomeric pads cannot be changed as a unit.
Furthermore, it is recognized that coil springs and elastomeric pads have compression response characteristics that are different. Since a coil spring has a constant spring rate and an elastomeric pad has a spring rate which increases as it is compressed, a coil spring will provide a linear increase in force per unit of compression, while an elastomeric pad will produce an exponential increase in force per unit of compression. Thus, for some riders (weight or skill level) and/or riding conditions, a coil spring might produce a more suitable ride while in others stacked elastomeric pads might prove more desirable. However, neither the Wilson et al. nor the Chen suspensions are designed to enable use of a coil spring instead of the rod-mounted pad arrangement, and while the Chen suspension is provided with a coil spring in addition to the rod-mounted pad arrangement, its function is primarily to allow the permissible degree of fork travel to be adjusted.
Thus, there is still a need for a shock absorbing system for a bicycle fork which can combine the advantages of both a spring type shock absorber and a hydraulic shock damping unit, that will enable the spring component to be exchanged and adjusted in a way that allows the performance curve type to be changed and a wide range of spring rate adjustments to be obtained for adapting the suspension to widely differing circumstances of terrain, rider weight and riding experience, yet, at the same time being practical and economic for use on bicycles by the average rider thereof.